As I understand current us ground based midcourse defense has a 50% success rate... in very controlled tests... So the idea that are going to magically scale that up to that > 99.9% number needed is pretty silly.
It is just not realistic when you are talking about defending against thousands of missiles plus you have to factor in all the penetration aids like decoys, chaff clouds, cooldown shrouds, radar jamming, mirvs, marvs, cruise missiles, hypersonic glide vehicles, nuclear powered underwater drones, stealth warheads, emps and anything else scientists invent in the future. Sorry MAD is not going anywhere.